This invention relates to drilling completely through oriented electromagnetic steel sheet in order to improve the watt-loss properties.
Core material of transformers and other electrical machinery has long been made from grain-oriented electromagnetic steel sheets. In these sheets, the metal grains are singly-oriented in the (110)[001] Goss-position, as expressed on the Miller index, where body center cubes are in the cube-on-edge position. These steel sheets are cold rolled, and annealed to recrystallize the grains and are usually made of "silicon-steel", i.e., contain from 1% to 4.5% silicon. A thin insulating film is usually applied to the surface of the sheets. These sheets have a direction of ease-of-magnitization in the direction of rolling.
The metal grains of these cold rolled, annealed steel sheets have ferromagnetic domains of large size, usually 5 mm to 25 mm across. The large magnetic domains result in watt-loss due mostly to "anomalous" eddy current loss, which can account for about 1/2 of the watt-loss at commercial frequencies, the rest being accountable to classical eddy current and hysteresis loss. A variety of methods have been used to decrease the width of magnetic domains within the metal crystal structure. Fiedler et al., in U.S. Pat. No. 3,647,575, teaches shallow grooving through the insulating film and metal sheet surface, transverse to the rolled direction after recrystallization annealing. Ichiyama et al., in U.S. Pat. No. 4,293,350, teaches brief laser pulse irradiation of the insulating film coated, finally annealed metal sheet surface, transverse to the rolled direction, to induce a small but significant substructure, in order to limit domain widths and improve core loss. Both of these processes damage the mill glass or other insulative coating on the sheet surface.
Neiheisel et al., in U.S. Pat. No. 4,456,812, teaches continuous laser beam scanning across the rolled direction of the insulating film coated, metal sheet surface, to subdivide magnetic domains without damaging the insulative coating. Krause et al., in U.S. Pat. No. 4,645,547, teach a somewhat similar process, and Miller, in U.S. Pat. No. 4,500,771, and Krause et al., in U.S. Pat. No. 4,535,218 first curve the width of the sheet.
Ichiyama et al., in U.S. Pat. No. 4,363,677, teaches laser-beam irradiation of finally annealed metal sheet, followed by formation of an insulating film on the sheet surface at temperatures of less than 600.degree. C., so that subdivision of the magnetic domains is not reversed. The laser beam irradiation regions can be in the form of continuous lines, broken lines, or spots. The spots, which do not penetrate deeply into the metal surface, have an area of not less than 10.sup.-5 mm.sup.2, with a diameter between 0.004 mm (0.15 mil) and 1 mm (39 mil). Similarly, Ichiyama et al., in U.S. Pat. No. 4,613,842, teaches the same size, laser formed continuous lines, broken lines, or spots, utilized on different components of transformer cores, where the pattern of the lines or spots may differ, depending on the placement of the component.
In both Ichiyama et al. Patent Specifications, the laser beam irradiation transverse to the direction of ease-of-magnetization causes generation of small projections, which form nuclei of magnetic domains having walls at a 90.degree. angle to the laser pattern across the width of the component. This laser treatment causes the domains of the grain-oriented electromagnetic steel sheet to be subdivided. As a result of the subdivision the watt-loss properties are reduced. In both of these Ichiyama et al. methods, the sheets tend to bow after laser treatment, sometimes requiring an additional heat flattening step.
All of these prior art methods reduce watt-loss at varying levels up to 15%. However, all appear to lose the advantage of the laser scribing and resultant domain refinement when subjected to a subsequent stress relief anneal at over 700.degree. C. Therefore, these processes can be utilized only for stacked transformer core applications. What is needed is a method to produce the same watt-loss reduction, but which survives a 700.degree. C. to 800.degree. C. stress relief anneal. It is one of the main objects of this invention to provide treated, electromagnetic steel sheet which will reduce watt-loss up to 14%, which will impart equal stress through the volume of the sheet with no sheet distortion or bowing, will not reduce space factor or insulative coating resistance, and which will survive a 700.degree. C. stress relief anneal.